The service allows you finding out who has called you, when the telephone line was busy, switched off or was outside the zone of coverage. By doing so, it ensures that an important call will not be missed.
This is a Voice over Internet Protocol (VoIP) and streaming media service based on the Session Initiation Protocol (SIP) by which Internet telephony service providers (ITSPs) deliver telephone services and unified communications to customers equipped with SIP-based private branch exchange (IP-PBX) and Unified Communications facilities